Biology · Photosynthesis in Higher Plants · NEET
Aerobic bacteria need oxygen to live and they move toward it. Engelmann had no oxygen sensor in 1880s. So he used the bacteria as living O2 detectors. Wherever the alga made the most O2 by photosynthesis, the bacteria crowded there. This is why they gathered under blue and red light. This clever detector idea is often asked in NEET.
Chlorophyll absorbs blue and red light strongly but reflects green light. Light that is absorbed drives photosynthesis and releases O2. So O2 was highest under blue and red, where bacteria gathered, and lowest under green. This is the exact answer NEET tests (blue and red light).
NCERT clearly states Engelmann used Cladophora, a green filamentous alga. Some other textbooks mention Spirogyra for a similar experiment, which causes confusion. For NEET, follow NCERT: the alga was Cladophora and the detector was aerobic bacteria.
It gave the first action spectrum of photosynthesis. It showed that photosynthesis (measured as O2 release) is highest in blue and red light and lowest in green. This action spectrum roughly matches the absorption spectrum of chlorophyll a and b, proving chlorophyll is the pigment that drives photosynthesis.
The prism split white light into its spectral colours (violet, blue, green, yellow, orange, red). This spread the colours along the length of the alga. Each part of the alga received a different colour, so Engelmann could see which colour caused the most O2 release. Without the prism, he could not compare colours.
One scientist cultured Cladophora in a suspension of aerobic bacteria and illuminated the culture by splitting light through a prism. He observed that bacteria accumulated mainly in the region of:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
T.W. Engelmann (1843-1909) performed it. He used a prism to split light onto the green alga Cladophora placed in a suspension of aerobic bacteria.
Photosynthesis is highest in blue and red light. This gave the first action spectrum of photosynthesis, which roughly matches the absorption spectrum of chlorophyll a and b.
Chlorophyll reflects green light instead of absorbing it. Light that is not absorbed cannot drive photosynthesis, so little O2 is made in green light and few bacteria gather there.
It directly measured the action spectrum (rate of photosynthesis, seen as O2 release, at each colour). This action spectrum resembles the absorption spectrum of chlorophyll.
Cladophora, a green filamentous alga. NEET follows NCERT, so always answer Cladophora, not Spirogyra.
